WDPN31 PGTW 030900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 04W (CHOI-WAN)
WARNING NR 019//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
TROPICAL STORM 04W (CHOI-WAN), LOCATED APPROXIMATELY 230NM
NORTHWEST OF MANILA, PHILIPPINES, HAS TRACKED NORTHWESTWARD AT 15
KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY (MSI) SHOWS A PARTIALLY-EXPOSED LOW-LEVEL CIRCULATION (LLC)
WITH BROAD CENTRAL CONVECTION OFFSET TO THE SOUTHWEST. THE INITIAL
POSITION IS PLACED WITH HIGH CONFIDENCE USING LOW CLOUD TRACING IN
THE MSI LOOP AND A TIMELY 030533Z ATMS 88.2GHZ IMAGE. THE INITIAL
INTENSITY OF 35KTS IS SET WITH FAIR CONFIDENCE BASED ON AN OVERALL
ASSESSMENT OF DVORAK ESTIMATES OF T2.0/30KTS (PGTW), T2.5/35KTS
(RJTD), T2.4/34KTS (ADT), AND SATELLITE CONSENSUS (SATCON) OF 42KTS,
REFLECTING THE DEEPENED STATE. THE SYSTEM IS IN A MARGINAL
ENVIRONMENT WITH GOOD WESTWARD AND EQUATORWARD OUTFLOW AND VERY WARM
(30-31C) SEA SURFACE TEMPERATURES, OFFSET BY MODERATE TO STRONG (20-
25KT) VERTICAL WIND SHEAR FROM THE NORTHEAST. THE CYCLONE IS
TRACKING ALONG THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE (STR)
TO THE EAST-NORTHEAST.
3. FORECAST REASONING.
A. THERE IS A CHANGE TO THE FORECAST PHILOSOPHY. THE FORECAST
TRACK HAS DECREASED TO TAU 72, INCLUDING EXTRA-TROPICAL TRANSITION.
B. TS 04W WILL CONTINUE TO TRACK NORTHERLY OVER THE NEXT 12
HOURS, THEN TURN NORTHEASTWARD AFTER TAU 12 UNDER THE STEERING
INFLUENCE OF THE STR. THE SYSTEM IS FORECAST TO REMAIN AT 35KTS
UNDER THE AFOREMENTIONED MARGINAL CONDITIONS AS IT ROUNDS THE RIDGE
AXIS. BY TAU 36, TS 04W WILL WEAKEN TO 30KTS DUE TO INCREASING VWS
ASSOCIATED WITH STRONG NORTHEASTERLY FLOW, LAND INTERACTION FROM THE
RUGGED TERRAIN OF TAIWAN, AND A SECONDARY STR ANCHORED OVER
SOUTHEASTERN CHINA. THE SYSTEM WILL REMAIN AT 30KTS AS IT PASSES
NORTH OF KADENA AIR BASE BEFORE TAU 48 AND UP TO TAU 72, AS IT
CONTINUES NORTHEASTWARD. CONCURRENTLY AT TAU 48, TS 04W WILL UNDERGO
EXTRA-TROPICAL TRANSITION (ETT) AS IT ENTERS THE BAROCLINIC ZONE AND
WILL COMPLETE ETT EAST OF THE RYUKU ISLANDS AND SOUTH OF HONSHU, BY
TAU 72. AFTER TAU 48, TS CHOI-WAN WILL ACCELERATE NORTHEASTWARD ON
THE POLEWARD SIDE OF THE STEERING STR. THERE IS A DISTINCT
POSSIBILITY THAT TS 04W WILL DISSIPATE BEFORE ETT. THE FORECAST
TRACK IS LAID IN GOOD AGREEMENT WITH MODEL CONSENSUS FOR THE FIRST
24 HOURS. AFTER TAU 24, THE FORECAST TRACK IS MORE TO THE RIGHT OF
FORECAST CONSENSUS, OFFSETTING NAVGEM, WHICH IS THE EXTREME LEFT
OUTLIER WITH A NORTHWARD TRACK TOWARD THE TAIWAN STRAIT. NUMERICAL
MODELS REMAIN IN TIGHT AGREEMENT, LENDING GOOD CONFIDENCE TO THE
EXTENDED PORTION OF THE JTWC TRACK FORECAST.//
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